1. Field of the Invention
This invention relates to sonar direction finding.
More particularly the invention relates to methods and apparatus for determining the bearing of a source of a sonar signal.
2. Description of Related Art
Various methods and apparatuses are known for detecting the signals emitted by a sonar signal source and utilising them to determine the bearing of the source. Typically such methods involve the combination of the signals produced by two or more detectors which are differently orientated with respect to the sonar signal source, e.g. three detectors giving cardioid response characteristics at 120xc2x0 angular spacing.
In general, such known methods require matching or knowledge of the directional characteristics of the detectors.
It is an object of the present invention to provide a method and apparatus for determining the bearing of a source of a sonar signal wherein this requirement is avoided.
According to a first aspect of the present invention there is provided a method of determining the bearing of a source of a sonar signal comprising determining the phase of a cyclic variation at a predetermined frequency of a chosen component of the output of a sonar detector monitoring said signal, said cyclic variation being due to rotation of the orientation of said detector with respect to the source at said predetermined frequency and said detector having a directional characteristic such that its output varies cyclically with the bearing of said source.
Preferably said phase is determined by separating from the output of said detector a component at said predetermined frequency and determining the phase of that component.
According to a second aspect of the present invention there is provided a method of determining the bearing of a source of a sonar signal comprising: monitoring the signal emitted by the source using a detector having a directional characteristic such that the amplitude of the signal produced by the detector varies with the bearing of the source; analysing the spectrum of the signal produced by the detector with the directional characteristic of the detector at a first orientation with respect to the source to obtain a signal representative of the amplitude of a chosen sinusoidal component of said signal produced by the detector; similarly obtaining signals representative of the amplitude of said component when the directional characteristic of the detector is at each of a plurality of further different orientations with respect to said source; arranging said signals representative of said amplitudes to form a time series of samples of the output of the detector relating to said component as the orientation of the detector is rotated at a predetermined frequency; and using said time series of samples to analyse the spectrum of the signal defined by said time series of samples and thereby determine the phase of a sinusoidal component thereof of a frequency determined by said rotation of the detector.
According to a third aspect of the invention there is provided an apparatus for determining the bearing direction of a source of a sonar signal comprising: a sonar signal detector having a directional characteristic such that the amplitude of the output of the detector varies with the bearing of the source; first processing means for deriving from the output of the detector a signal representing the amplitude of a chosen sinusoidal component of the output of the detector; means for positioning the directional characteristic of the detector at each of a plurality of orientations with respect to said source; and second processing means for processing the signals produced by said first processing means at each orientation of the detector as a time series of samples of the output of the detector at the frequency of said component as the orientation of the detector is rotated at a predetermined frequency, thereby to determine the phase of a sinusoidal component of the signal defined by said time series of a frequency determined by said rotation of the detector.
In a preferred apparatus according to the invention said first and second processing means are one and the same processing means.